The present invention relates to a seat belt tensioning device to be mounted in vehicles such as automobiles, particularly to a seat belt tensioning device of a type such that a piston connected to a seat belt is inserted within a cylinder.
The seat belt tensioning device of this type is provided with a piston operating means which moves the piston within the cylinder at an urgent time, such as, upon a vehicle impact.
When the piston is moved to tension the seat belt, the passenger is held onto his seat in the vehicle. For the piston operating means, gas pressure or the like is utilized.
It is very difficult to adjust the tension force of the seat belt (that is, a piston stroke) into a desired value precisely by a piston operating means utilizing gas pressure or the like. In addition a constant tension force cannot be always obtained.
If the tension force of the seat belt is small, the passenger is in danger of the secondary impact. On the other hand, if the tension force becomes too large, he is in danger of being injured by the seat belt which is tensioned by such a large tension force.
Also in one example of the seat belt tensioning device of this type, the seat belt is connected to the piston which is disposed within the cylinder through a deformable member which is plastically deformed due to a predetermined load.
Upon a vehicle impact, the seat belt is elongated by the deformable member to such an extent that the passenger does not collide against the front panel of the vehicle and the like secondly. The kinetic energy of the passenger is absorbed in the elongating process of the seat belt.
However, in the above described conventional seat belt tensioning device, if the tension force of the seat belt, which is very difficult to adjust, exceeds the plastically deforming load, the deformable member is started to be elongated in a direction reverse to the piston moving direction while the piston is being moved by the piston operating means.
As a result, the desired stroke of the deformable member for absorbing the kinetic energy of the passenger cannot be obtained.
If the tension force of the seat belt is too small, the deformable member begins to be elongated before the piston is moved by a required stroke. Also the passenger becomes more endangered since he is apt to encounter by the secondary impact.
Accordingly, an object of the present invention is to provide a seat belt tensioning device by which a tension force of the seat belt, which is generated due to the movement of the piston, can be adjusted to a desired value precisely.
Another object of the present invention is to provide a seat belt tensioning device provided with a deformable member which begins to be elongated when the tension force of the seat belt reaches a predetermined value, and which effectively absorbs the kinetic energy of the passenger.